CMOS technology is receiving increasing attention for radio frequency and millimeter wave (mmWave) applications due to integration of digital processors and memory circuits with RF circuits. Cut-off frequency (fT) and maximum oscillation frequency (fMAX) are recognized as performance figures-of-merit for semiconductor devices. fT is the frequency at which the forward small-signal, short-circuit current gain of an active device has a value of unity. fMAX is the frequency at which the power gain of the device falls to unity. As the frequency of operation of ICs moves closer to the peak fT and fMAX of the technology, layout optimization becomes crucial to the design.
As is well known, fT and fMAX for a CMOS device are mainly limited by the parasitic components associated with the devices gate, source and drain connections in addition to the intrinsic transit time from source to drain across the device channel. The most significant parasitics are the gate-source capacitance (Cgs), gate-drain capacitance (Cgd) and gate resistance (Rg). For an FET, FIG. 1A is a top view 100 of the FET, FIG. 1B is a cross-section view 120 of the FET, and FIG. 1C illustrates a simplified equivalent circuit schematic 140 of the FET. The views 100, 120 and 140 show the parasitic capacitances Cgs 122, Cgd 123 and Cds 124. Equivalent circuit 140 and view 100 also show the gate resistance (Rg) 141. In FIG. 2, equations 1 and 2 show the fundamental theoretical relationship between the parasitic components and the FET figures of merit. Each parasitic element in these equations consists of a component intrinsic to the device and an extrinsic component related to its metal interconnections.
Contemporary ICs, such as input-out drivers or power amplifiers, face multiple challenges. They operate at RF and mmWave frequencies while amplifying, receiving or delivering signals with voltage amplitudes comparable to or higher than the power supply of the circuit, and/or a power in the order of milliwatts or higher. Connections to devices in ICs, for example, connections to an FET device, will operate at relatively large current densities and may face electromigration (EM) reliability issues. When electrical current flows through metal conductors or interconnections, EM or current limiting mechanisms due to heat may occur, causing failure of ICs. EM is due to metal atoms being pushed, in proportion to the current density, by the electron flow comprising the current. This may cause disconnection of the interconnection or an open circuit. As the cross-section of metal interconnects becomes smaller with technology scaling, current density becomes larger for a given amount of current being carried, and EM becomes more severe.
An EM compliant FET usually requires a larger amount of metallization. This adds parasitic capacitance and resistance, degrading the device cut-off frequency (fT) and maximum oscillation frequency (fMAX). An FET that must handle large signals at high frequencies faces the challenge of both, being EM compliant with respect to high direct current and high alternating current, and attaining the highest possible fT and fMAX.
As is known in the art, ICs usually are fabricated to include a plurality of metal conductor layers, typically up to 8. The metal layers are interconnected using vias. In ICs operating at RF and mmWave frequencies, the connections between devices and sub-circuits to communicate electrical signals are usually implemented with transmission line structures formed by a conductor at a high metal level and a ground plane at a low metal level. The high metal levels are usually thicker, therefore providing lower resistance, and are a longer distance from lower metal levels, where the ground plane usually resides. Both characteristics are desirable for transmission line design. An effective layout design for an FET operating at RF and mmWave frequencies must take into account all of the metal levels and vias required for its connection to the highest metal levels of the technology, where transmission lines will carry the signals related to the device.
In U.S. Pat. No. 7,132,717, the disclosure of which is incorporated herein by reference, a power metal oxide semiconductor transistor layout is disclosed. The power metal oxide semiconductor transistor layout uses network of conductive lead line as a connection or a network connection to connect source and drain regions thereby achieves advantages of a high uniformity of current, low Rds_on, much less power loss, an actual line density two times larger than that of conventional layouts and a strengthened resistance to electron migration.
In U.S. Pat. No. 6,958,541, the disclosure of which is incorporated herein by reference, a region on a substrate contains multiple transistors in parallel that share a single salicided polysilicon gate electrode. Above or below the gate electrode are formed multiple plugs of refractory material along the length of the gate electrode. The multiple plugs of refractory material electrically interconnect the gate signal line and the salicided polysilicon gate electrode. The plug material is selected to minimize the work function between it and the salicided polysilicon gate electrode.